Plants routinely become infected by fungi and bacteria, and many microbial species have evolved to utilize the different niches provided by the growing plant. Some phytopathogens have evolved to infect foliar surfaces and are spread through the air, from plant-to-plant contact or by various vectors, whereas other phytopathogens are soil-borne and preferentially infect roots and newly germinated seedlings. In addition to infection by fungi and bacteria, many plant diseases are caused by nematodes which are soil-borne and infect roots, typically causing serious damage when the same crop species is cultivated for successive years on the same area of ground.
Plant diseases cause considerable crop loss from year to year resulting both in economic hardship to farmers and nutritional deprivation for local populations in many parts of the world. The widespread use of fungicides has provided considerable security against phytopathogen attack, but despite $1 billion worth of expenditure on fungicides, worldwide crop losses amounted to approximately 10% of crop value in 1981 (James, Seed Sci. & Technol. 9:679-685 (1981). The severity of the destructive process of disease depends on the aggressiveness of the phytopathogen and the response of the host, and one aim of most plant breeding programs is to increase the resistance of host plants to disease. Novel gene sources and combinations developed for resistance to disease have typically only had a limited period of successful use in many crop-pathogen systems due to the rapid evolution of phytopathogens to overcome resistance genes. In addition, there are several documented cases of the evolution of fungal strains which are resistant to particular fungicides. As early as 1981, Fletcher and Wolfe (Proc. 1981 Brit. Crop Prot. Conf. (1981)) contended that 24% of the powdery mildew populations from spring barley, and 53% from winter barley showed considerable variation in response to the fungicide triadimenol and that the distribution of these populations varied between barley varieties with the most susceptible variety also giving the highest incidence of less susceptible fungal types. Similar variation in the sensitivity of fungi to fungicides has been documented for wheat mildew (also to triadimenol), Botrytis (to benomyl), Pyrenophora (to organomercury), Pseudocercosporella (to MBC-type fungicides) and Mycosphaerella fijiensis to triazoles to mention just a few (Jones and Clifford; Cereal Diseases, John Wiley, 1983). Diseases caused by nematodes have also been controlled successfully by pesticide application. Whereas most fungicides are relatively harmless to mammals and the problems with their use lie in the development of resistance in target fungi, the major problem associated with the use of nematicides is their relatively high toxicity to mammals. Most nematicides used to control soil nematodes are of the carbamate, organochlorine or organophosphorous groups and must be applied to the soil with particular care.
In some crop species, the use of biocontrol organisms has been developed as a further alternative to protect crops. Biocontrol organisms have the advantage of being able to colonize and protect parts of the plant inaccessible to conventional fungicides. This practice developed from the recognition that crops grown in some soils are naturally resistant to certain fungal phytopathogens and that the suppressive nature of these soils is lost by autoclaving. Furthermore, it was recognized that soils which are conducive to the development of certain diseases could be rendered suppressive by the addition of small quantities of soil from a suppressive field (Scher et al. Phytopathology 70:412-417 (1980). Subsequent research demonstrated that root colonizing bacteria were responsible for this phenomenon, now known as biological disease control (Baker et al. Biological Control of Plant Pathogens, Freeman Press, San Francisco, 1974). In many cases, the most efficient strains of biological disease controlling bacteria are of the species Pseudomonas fluorescens (Weller et al. Phytopathology 73:463-469 (1983); Kloepper et al. Phytopathology 71:1020-1024 (1981)). Important plant pathogens that have been effectively controlled by seed inoculation with these bacteria include Gaemannomyces graminis, the causative agent of take-all in wheat (Cook et al. Soil Biol. Biochem 8:269-273 (1976)) and the Pythium and Rhizoctonia phytopathogens involved in damping off of cotton (Howell et al. Phytopathology 69:480-482 (1979)). Several biological disease controlling Pseudomonas strains produce antibiotics which inhibit the growth of fungal phytopathogens (Howell et al. Phytopathology 69:480-482 (1979); Howell et al. Phytopathology 70:712-715 (1980)) and these have been implicated in the control of fungal phytopathogens in the rhizosphere. Although biocontrol was initially believed to have considerable promise as a method of widespread application for disease control, it has found application mainly in the environment of glasshouse crops where its utility in controlling soil-borne phytopathogens is best suited for success. Large scale field application of naturally occurring microorganisms has not proven possible due to constraints of microorganism production (they are often slow growing), distribution (they are often short lived) and cost (the result of both these problems). In addition, the success of biocontrol approaches is also largely limited by the identification of naturally occurring strains which may have a limited spectrum of efficacy. Some initial approaches have also been taken to control nematode phytopathogens using biocontrol organisms. Although these approaches are still exploratory, some Streptomyces species have been reported to control the root knot nematode (Meliodogyne spp.) (WO 93/18135 to Research Corporation Technology), and toxins from some Bacillus thuringiensis strains (such as israeliensis) have been shown to have broad anti-nematode activity and spore or bacillus preparations may thus provide suitable biocontrol opportunities (EP 0 352 052 to Mycogen, WO 93/19604 to Research Corporation Technologies).
The traditional methods of protecting crops against disease, including plant breeding for disease resistance, the continued development of fungicides, and more recently, the identification of biocontrol organisms, have all met with success. It is apparent, however, that scientists must constantly be in search of new methods with which to protect crops against disease. This invention provides novel methods for the protection of plants against phytopathogens.